1. Field of the Invention
This invention relates to an image pickup apparatus using an image pickup device such as CCD (Charge Coupled Device), and especially relates to a light quantity control system for exposing the image pickup device.
2. Description of the Related Art
This type of image pickup apparatus is described below by taking a digital camera as an example.
In recent years, a density of pixels of the CCD becomes higher, so that a ½ inch sized CCD with 1.5 million pixels is realized. In a conventional digital camera at initial stage, a diameter of an aperture of an optical system is mechanically controlled for adjusting a quantity of exposing light similar to a conventional camera system using a silver-salt film. Alternatively, a time period for storing electric charge (hereinafter abbreviated as charge storage time) in each pixel of the CCD is controlled for adjusting a quantity of exposing light under a fixed diameter of the aperture of the optical system.
A relation between the optical system and the diameter of the aperture thereof in the digital camera with the ½ inch sized CCD (hereinafter abbreviated as digital camera) is compared with another relation between them in a conventionally popularized camera using a 135 size silver-salt film (hereinafter abbreviated as 135 size camera).
A frame size of the 135 size camera is 24 mm×36 mm, so that a length of a diagonal of the frame is about 43 mm. With respect to the frame size, a focal length of a standard lens of the 135 size camera is, for example, 50 mm. When this relation is applied to the digital camera, since a length of a diagonal of the frame of the digital camera is ½ inch (about 13 mm), a focal length of a standard lens becomes about 15 mm. An aperture number (F number) of the lens is a result that the focal length is divided by the diameter of an aperture diaphragm. When the aperture number is F16 in the digital camera, the diameter of the aperture diaphragm becomes smaller than 1 mm. It is substantially pinhole. The shorter the focal length of the lens becomes, the smaller the diameter of the aperture diaphragm becomes.
When the diameter of the aperture diaphragm of the lens becomes smaller, it is difficult mechanically to control the diameter of the aperture diaphragm accurately. Furthermore, quality of an image is deteriorated by diffraction. Especially, in the digital camera with a high pixel-density CCD, the size of each pixel becomes much smaller, so that the deterioration of the quality of the image due to the diffraction cannot be disregarded.
In order to control the aperture number not to be larger than a predetermined value, in other words, to control the diameter of the aperture diaphragm not to be smaller than a predetermined value, it is proposed to use an ND (Neutral Density) filter which is an optical light quantity control device is used together with the aperture diaphragm which is a mechanical light quantity control device. For example, a publication gazette of Japanese Patent Application Hei 10-210487 shows a conventional digital camera with an ND filter inserted into an optical path of an image pickup optical system by movement of a plunger when a luminance of an object is higher than a predetermined value. In the conventional digital camera, a white filter is further inserted into the optical path by the plunger for adjusting white balance of an image automatically. When an image data of the white filter is taken, output levels of signals with respect to R (Red), G (Green) and B (Blue) are balanced by using the image data.
The digital camera is generally driven by an energy from a battery. The CCD and an LCD (Liquid Crystal Display) used in the digital camera, however, consume a lot of electric energy. Thus, the digital camera conventionally has a problem that the energy of the battery is consumed fast. In the above-mentioned conventional digital camera shown in the publication gazette Hei 10-210487, the ND filter is automatically inserted into the optical path by the movement of the plunger when the luminance of the object is higher than the predetermined value, even though the digital camera is in a waiting state of the image pickup operation. The electric energy of the battery is largely consumed by not only the CCD and the LCD, but also the plunger. The life of the battery becomes much shorter.
On the other hand, the ND filter is an optical parallel, so that the optical path length is changed when the ND filter is inserted into the optical path, and the position of the image (or focal plane) moves backward. Thus, when the focusing of the taking lens is completed before the ND filter is inserted into the optical path, the image on the CCD is defocused due to insertion of the ND filter. In the digital camera at initial stage with the CCD having three hundred thousand pixels, the density of the pixels of the CCD is relatively smaller and the resolution of the image is lower. Thus, the defocus of the image due to the insertion of the ND filter can be disregarded. However, when the number of the pixels of the CCD becomes larger than one million, the resolution of the image becomes much higher, so that the defocus of the image due to the insertion of the ND filter cannot be disregarded. This phenomenon of the defocus becomes noticeable when a focal length of a zoom lens is changed to be longer.
The ND filter primarily cuts the quantity of light evenly by the same ratio with no relation to the wavelength of the light. Such an ideal ND filter, however, is impossible to produce. An actual ND filter has variation of the spectral transmittance as shown in FIG. 8. In the example of the ND filter shown in FIG. 8, the transmittance of red is higher, so that the light passing through the ND filter is tinged with red. When the white balance is adjusted before the insertion of the ND filter, the levels of the signals of R, G and B from the CCD becomes imbalance due to the insertion of the ND filter.
There is no problem of the above-mentioned defocus of the image or the imbalance of the white balance due to the insertion of the ND filter, when the ND filter is inserted into the optical path just before the image pickup operation, and the taking lens is focused and the white balance is adjusted after the insertion of the ND filter. However, when the ND filter and the white filter are inserted into the optical path by the movement of the plunger, a time period for supplying the electric energy to the plunger becomes longer and the battery is consumed fast. Furthermore, a time lag from pushing on of a shutter release switch to an actual image pickup by the CCD becomes much longer. User misses a chance for taking a good picture image.